Melt spinning technologies are routinely employed to fabricate nonwoven webs and multilayer laminates or composites, which are manufactured into various consumer and industrial products, such as cover stock materials for single-use or short-life absorbent products, disposable protective apparel, fluid filtration media, and durables including bedding and carpeting. Melt spinning technologies, including spunbonding processes and meltblowing processes, form nonwoven webs and composites from one or more layers of intertwined filaments or fibers, which are composed of one or more thermoplastic polymers. Fibers formed by spunbonding processes are generally coarser and stiffer than meltblown fibers and, as a result, spunbonded webs are generally stronger but less flexible than meltblown webs.
A meltblowing process generally involves extruding a row of fine diameter, semi-solid filaments of one or more thermoplastic polymers from a meltblowing die of a melt spinning apparatus and attenuating the extruded filaments while airborne with high velocity, heated process air immediately upon discharge from the melt spinning apparatus. The process air may be discharged as continuous, converging sheets or curtains on opposite sides of the discharged filaments or as individual streams or jets associated with the filament discharge outlets. The attenuated filaments are then quenched with a flow of a relatively cool process air and blown in a filament/air mixture for depositing in a forming zone to form a meltblown nonwoven web on a collector, such as a substrate, a belt or another suitable carrier, moving in a machine direction.
A spunbonding process generally involves extruding multiple rows of fine diameter, semi-solid filaments of one or more thermoplastic polymers from an extrusion die of a melt spinning apparatus, such as a spinneret or spinpack. A voluminous flow of relatively cool process air is directed at the stream of extruded filaments to quench the molten thermoplastic polymer. A high-velocity flow of relatively cool process air is then used to attenuate or draw the filaments to a specified diameter and to orient them on a molecular scale. The process air is heated significantly by thermal energy transferred from the immersed filaments. The attenuated filaments are propelled in a filament/air mixture toward a forming zone to form a nonwoven web or a layer of a laminate on a moving collector.
Spunbonding processes typically incorporate a filament drawing device that provides the high velocity flow of process air for attenuating the filaments. Hydrodynamic drag due to the high velocity air flow accelerates each filament to a linear velocity or spinning speed significantly greater than the speed of extrusion from the extrusion die and applies a tensile force that attenuates the filaments as they travel from the die to the inlet of the filament drawing device. Some additional attenuation occurs between the outlet of the filament drawing device and the collector as the filaments are entrained by the high velocity air exiting the filament drawing device. Conventional filament drawing devices accelerate the filaments to an average linear velocity less than 8000 meters per minute (m/min).
One deficiency of conventional filament drawing devices is that a large volume of high velocity process air is required for attenuating the filaments. In addition, the process air captures or entrains an excessive volume of secondary air from the ambient environment surrounding the airborne filament/air mixture. The volume of entrained secondary air is proportional to the volume and velocity of the process air exiting the filament drawing device. If left unmanaged, such large volumes of high velocity process and secondary air tend to disturb the filaments as they deposit on the collector, which degrades the physical properties of the spunbonded web.
As mentioned above, large volumes of process air are generated during both the meltblowing and spunbonding processes. Moreover, much of the process air is heated and is moving with high velocities, sometimes approaching sonic velocities. Without properly collecting and disposing of the process air and the entrained secondary air, large volumes of high-speed air would likely disturb personnel working around the manufacturing apparatus and other nearby equipment. Further, large volumes of heated process air would likely heat the surrounding area in which the nonwoven web or laminate is being fabricated. Consequently, attention must be paid to collecting and disposing of this process air and entrained secondary air when manufacturing nonwoven webs and laminates with melt spinning technologies.
Management of the process and secondary air is also important with regard to tailoring the characteristics of the filaments as deposited on the moving collector. The homogeneity of the distribution of deposited filaments across the width of the nonwoven web, or in the cross-machine direction, depends greatly on the uniformity of the air flow in the cross-machine direction around the filaments as they are deposited onto the collector belt. If distribution of air flow velocities in the cross-machine direction is not uniform, the filaments will not be deposited onto the collector uniformly, yielding a nonwoven web that is nonhomogeneous in the cross-machine direction. Thus, the variation of the air flow velocity in the cross-machine direction should be minimized in order to produce a nonwoven web having homogenous physical properties, such as density, basis weight, wettability, and fluid permeability, in the cross-machine direction. Moreover, large volumes of unmanaged air may also affect fiber formation upstream and downstream of the forming zone in the upstream and downstream fiber-making beams, respectively. Therefore, effective and efficient disposal of large volumes of air is necessary to avert irregularities in the physical properties of the nonwoven web.
Filaments deposited onto the collector have an average fiber orientation in the machine direction (MD) and an average fiber orientation in the orthogonal cross-machine direction (CD). The ratio of filament orientation, termed the MD/CD laydown ratio, indicates the isotropicity of the nonwoven web and strongly influences various properties of the nonwoven web, including the directionality of the tensile strength or flexibility of the web. Given a uniform distribution of air flow velocities in the cross-machine direction, the distribution of air flow velocities in the machine direction controls the MD/CD laydown ratio and, therefore, is an important consideration in the management of the large volumes of process and secondary air.
Various conventional air management systems have been used to collect and dispose of the flow of process and secondary air generated by melt spinning apparatus. Most conventional air management systems include an air moving device, such as a blower or vacuum pump, and a collecting duct having an intake opening positioned below the collector proximate to the forming zone for collecting the air and an exhaust opening coupled in fluid communication with the air moving device for disposing of the collected air. In some of these conventional systems, the negative pressure applied at the intake opening is controlled by one or more movable dampers positioned at the threshold of the intake opening. In other conventional air management systems, the collecting duct is subdivided into an array of smaller air passageways in which each individual air passageway includes an intake opening, an exhaust opening, and an air moving device coupled in fluid communication with the exhaust opening for drawing the collected air into the individual intake openings. Control of the negative air pressure applied at the intake opening is provided by multiple moveable dampers each associated with an exhaust opening of one of the air passageways.
Controlling the distribution of air flow velocities proximate to the forming zone in both the cross-machine and machine directions simultaneously, however, has proven challenging for conventional air management systems. Conventional air management systems, such as those described above, are incapable of systematically controlling the directionality or symmetry of the air flow velocities in the machine direction while maintaining a relatively uniform distribution of air flow velocities in the cross-machine direction. In particular, movable dampers in such conventional systems either are incapable of varying the distribution of air flow velocities in the machine direction or cannot vary the distribution of air flow velocities in the machine direction without significantly reducing the uniformity of the air flow velocities in the cross-machine direction. As a result, conventional air management systems lack the ability to select the distribution of air flow velocities in the machine direction in order to effectively control the MD/CD laydown ratio. It follows those melt spinning processes using such conventional air management systems cannot control or otherwise tailor the properties of the nonwoven web in the machine direction.
What is needed, therefore, is a method for manipulating the disposal of the process air so as to control the distribution of air flow velocities near the forming zone for the nonwoven web in the machine direction and maintain a uniform air flow in the cross-machine direction.